Circadian clocks are endogenous timekeeping mechanisms found in organisms ranging from cyanobacteria to humans. Clock function coordinates an organism's molecular, cellular, and behavioral rhythms with the exogenous rhythms of light, temperature, and humidity caused by the Earth's planetary motions. Rhythm coordination requires regulatory input, central-clock function, and an adaptive output. For humans, understanding circadian clock function may improve treatment of many sleep disorders, may lead to reductions in work- and travel-related accidents caused by morning drowsiness, and may eventually allow clinical treatments to be coordinated with a patients natural biological rhythms. Three genes, kaiA, kaiB and kaiC, from a putative circadian- clock locus in Synechococcus sp. strain PCC 7942 were recently cloned, sequenced and characterized. This marks an entry point for research into the mechanism and regulatory controls of circadian clocks using a prokaryotic model. Proposed experiments take advantage of the relative simplicity of molecular genetics in Synechococcus to begin developing a mechanistic circadian-clock model. Experiments address clock function by examining regulation of kai gene expression. Experiments are also proposed that explore specific interactions among kai gene products and between kai gene products and other Synechococcus proteins. A prokaryotic clock model should provide a foundation for understanding clock function in other, more complex, organisms.